1. Field of the Invention
The invention relates to a nitride semiconductor template and a light-emitting diode using the nitride semiconductor template.
2. Description of the Related Art
Nitride-based compound semiconductors such as gallium nitride (GaN), aluminum gallium nitride (AlGaN) and indium gallium nitride (InGaN) have received attention as a light-emitting element material capable of emitting light from red to ultraviolet. One of crystal growth methods of such nitride semiconductor materials is a Hydride Vapor Phase Epitaxy (HVPE) method in which metal chloride gas and ammonia are used as raw materials.
The HVPE method is characterized in that it is possible to obtain a growth rate of not less than 10 μm/hr and not more than 100 μm/hr which is much faster than a typical growth rate of several μm/hr obtained by other growth methods (Metal-Organic Vapor Phase Epitaxy (MOVPE) method or Molecular Beam Epitaxy (MBE) method). Therefore, the HVPE method is often used for manufacturing a GaN free-standing substrate (see Japanese Patent Application No. 3886341) or an AlN free-standing substrate. The “free-standing substrate” here refers to a substrate which can maintain its shape and has a strength which does not cause inconvenience for handling.
Meanwhile, a light-emitting diode (LED) made of nitride semiconductor is generally formed on a sapphire substrate. For the crystal growth thereof a buffer layer is formed on a surface of the sapphire substrate, a thick GaN layer of about 10 to 15 μm including an n-layer is subsequently grown thereon, and then, a light-emitting layer of InGaN/GaN multiple quantum well (total thickness of several hundred nm) and p-layer (thickness of 200 to 500 nm) are grown thereon in this order. The reason why the GaN layer under the light-emitting layer is thick is, e.g., to improve crystalline characteristics of GaN on the sapphire substrate. After that, electrodes, etc., are formed, thereby eventually forming a below-described element structure shown in FIG. 4. In case of growing by the MOVPE method, the crystal growth process typically takes about 6 hours, about half of which is time required for growing the GaN layer, called template, formed under the light-emitting layer.
Based on the above, it is possible to significantly reduce growth time and thus to drastically reduce the manufacturing cost of LED wafer if the HVPE method providing a considerably fast growth rate can be applied to growth of a template portion.
Meanwhile, for example, JP-A2002-280611 is a method of improving light extraction efficiency by reducing optical confinement in a semiconductor light-emitting element (light-emitting diode).
In JP-A 2002-280611, a first layer is embossing-processed to increase light extraction efficiency, thereby enhancing brightness. Meanwhile, in JP-A 2005-183997, the same effect is obtained by an embossing process on a substrate. In JP-A 2005-183997, so called PSS (Patterned Sapphire Substrate) is used to increase light extraction efficiency and thereby to enhance brightness.
Meanwhile, the template portion needs to have low resistance since it is a portion where electric current flows in a horizontal direction. This is required because drive voltage (forward voltage) of LED increases unless resistance is low. That is, the template portion is an important portion which has a function of reducing defects in an active layer by improving crystalline characteristics to enhance internal quantum efficiency and a function of reducing forward voltage.